Filter circuits using alternate openand short-circuited 3 db quadrature hybrids



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C 2 1y 4 M 3 r N Hv A 4 x0 $3 my I v m i l L M w rm 1 5. m r A ofwh ATTORNEV United States Patent 3,500,259 FILTER CIRCUITS USING ALTERNATEOPEN AND SHORT-CIRCUITED 3 db QUADRATURE HYBRIDS Harold Seidel, WarrenTownship, Somerset County, N.J., assignor to Bell TelephoneLaboratories, Incorporated, Murray Hill and Berkeley Heights, N.J., acorporation of New York Filed Sept. 29, 1967, Ser. No. 671,649 Int. Cl.H03h 5/06, 7/10 US. Cl. 33326 4 Claims ABSTRACT OF THE DISCLOSURE Theinvention described in this application is based upon the recognitionthat an open-circuited 3 db quadrature hybrid is equivalent to one ormore suitably spaced, series-connected circuits, series resonant at the3 db coupling frequency of the hybrid, wher as a short-circuited 3 dbquadrature hybrid is equivalent to one or more suitably Spacedshunt-connected circuits, parallel resonant at the hybrids 3 db couplingfrequency. As such, entire filter structures can be constructed usingsolely opencircuited and short-circuited 3 db quadrature couplers.

Because the phase of the output signal from an opencircuited couplerdifiers by 180 degrees from that of a short-circuited coupler, two dualfilters, each containing an odd number of couplers, can be emp oyed as atransformer for coupling an unbalanced signal source to a balanced load.

This invention relates to filter circuits using opencircuited andshort-circuited 3 db quadrature hybrids.

BACKGROUND OF THE INVENTION It is customary to think of circuits interms of inductors, capacitors and resistors. These components, inaddition to being the traditional circuit components, can be madesufficiently small and inexpensively to permit their use in largenumbers. Recently, other circuit components, such as the quadraturehybrid junction, have been developed to such a state where they too canbe made very small and inexpensively thus providing the circuit designerwith an additional, basic circuit component having a variety ofinteresting and useful properties.

SUMMARY OF THE INVENTION The present invention is based upon therecognition that an open-circuited 3 db quadrature coupler is equivalentto one or more, suitably spaced, series-connected circuits, seriesresonant at the 3 db coupling frequency of the hybrid, whereas ashortcircuited quadrature hybrid is equivalent to one or more, suitablyspaced, shunt-conuected circuits, parallel resonant at the hybrids 3 dbcoupling frequency.

In accordance with the invention, open-circuited and short-circuitedquadrature couplers are connected in series to produce a multi-sectionband-pass filter. The impedance of the respective equivalent filtersections is determined by the characteristic impedance of the hybridcoupler. Thus, it is a relatively simple matter to design filtersections having arbitrary pass-band and impedancematchingcharacteristics, using neither capacitors nor inductors, as such, merelyby appropriately selecting the characteristic impedanoes and the centerfrequencies of the quadrature hybrids making up the filter.

Because the phase of the output signal from an opencircuited couplerdiffers by 180 degrees from that of a short-circuited coupler, two dualfilters, each containing an odd number of couplers, can be employed as atransformer for coupling an unbalanced signal source to a balanced load.

3,500,259 Patented Mar. 10, 1970 ice BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 shows, in block diagram, an open-circuited, quadrature hybridjunction;

FIGS. 2A and 2B, included for purposes of explanation, show the signalsas a function of frequency at the several branches of an open-circuitedlumped-element hybrid, and an open-circuited transmission line typehybrid, respectively;

FIGS. 3A and 3B show the lumped-element and the transmission lineequivalent circuits of the hybrid of FIG. 1;

FIG. 4 shows, in block diagram, a short-circuited quadrature hybridjunction;

FIGS. 5A and 5B show the lumped-element and the transmission lineequivalent circuits of the hybrid of FIG. 4;

FIG. 6 shows a filter using a plurality of openand short-circuitedhybrids, alternately connected in series;

FIG. 7 shows the dual of the filter shown in FIG. 6; and

FIG. 8 shows an arrangement for obtaining a balanced output signal froman unbalanced signal source.

DETAILED DESCRIPTION Referring to the drawings, a quadrature hybridjunction 10 is represented in FIG. 1. The term quadrature hybridjunction (or quadrature coupler), is used in its accepted sense todescribe a power-dividing network having four branches (or ports) inwhich the branches are arranged in pairs, with the branches comprisingeach pair being conjugate to each other but in coupling relationship tothe branches of the other of said pairs. In particular quadraturecouplers are characterized by a scattering matrix which is symmetricalwith respect to both diagonals and independent of the order in which thebranches are selected. This includes a large variety of couplers such asthe Riblet coupler (H. J. Riblet, The Short-Slot Hybrid Junction,Proceedings of the Institute of Radio Engineers, February 1952, pages180-184), the multihole directional coupler (S. E. Miller, Coupled WaveTheory and Waveguide Applications, Bell System Technical Journal, May1954, pages 661-719), the semi-optical directional coupler (E. A. J.Marcatili, A Circular Electric Hybrid Junction and Some Channel-DroppingFilters, Bell System Technical Journal, January 1961, pages 185496), thestrip transmission line directional coupler (T. K. Shimizu Strip-line 3db Directional Coupler, 1957 Institute of Radio Engineers, WesconConvention Record, vol. 1, part 1, pages 4-15), and the lumped-elementquadrature hybrids sold by Merrimac Research and Development, Inc., asadvertised, for example, in the September 1966 issue of MicrowaveJournal. In each of the above-mentioned power dividers, there is adegree relative phase difference betwen the two output signalcomponents, hence the designation quadrature coupler or quadraturehybrid. In addition, the hybrids of interest are those which divide theincident power into two equal components at some center frequency. Suchhybrids are characterized as 3 db hybrids. Accordingly, to avoidunnecessary repetition hereafter, the use herein of such terms as hybridor coupler shall be understood to refer, more specifically, to a 3 dbquadrature hybrid junction.

Referring again to FIG. 1, the four branches of hybrid 10 are designated1, 2, 3 and 4, of which branches 1 and 2 constitute one pair ofconjugate branches, and branches 3 and 4 the other pair. In operation,an input 3 signal, coupled to branch 1, is divided into two quadraturecomponents Z and To in branches 3 and 4, respectively. For a lumped-elment hybrid the variations in amplitude of these two signal components,as a function of frequency, are represented in FIG. 2 by curves 11 and12. Basically, the 1 component is a maximum at the lower frequencies andfalls off as the frequency increases, whereas the k component is minimumat the lower frequencies, and increases as the frequency increases. Thetwo are equal in amplitude at some intermediate frequency f,,.

With branches 3 and 4 open-circuited, as illustrated in FIG. 1, thesignal components in branches 3 and 4 are reflected back toward branches1 and 2. The portion of the t component coupled to branch 1 is equal toWhile the portion coupled to branch 2 is equal to where I is thecoeflicient of reflection. Assuming a perfect open circuit, I is equalto one, and will be omitted hereafter.

Similarly, the 1? component couples a component I5 to branch 1 and acomponent is t to branch 2. Thus, the total reflected signal is equal toin branch 1 and in branch 2. Recognizing that at frequency f,,, ltl lki,and that Z and it are 90 degrees out of phase, the reflected signal inbranch 1 is zero and all the signal is coupled to branch 2. This, ofcourse, is one of the well-known properties of a 3 db quadraturecoupler.

If the reflected signal,

is examined as a function of frequency, (curve 13, FIG. 2A), it is seento have a typical resonance characteristic, peaking at center frequencyf,,, and decreasing as the frequency deviates from f In fact, it canreadily be shown that with respect to branches 1 and 2, theopen-circuited quadrature hybrid illustrated in FIG. 1 is equivalent toa series-connected circuit, series resonant at frequency f For thelumped-element quadrature hybrid, the equivalent circuit, shown in FIG.3A, comprises a series-connected inductor L and capacitor C, where L andC are related by where Z is the characteristic impedance of the hybrid.

FIG. 2B shows the variations of k, t and the product 21a, as a functionof frequency, for a transmission line type quadrature coupler. It willbe noted that the product curve 14 tends to be flatter than curve 13 inFIG. 2A. In this connection, it can be shown that this response isequivalent to a multiple-resonant circuit, such as shown in FIG. 3B,comprising two quarter-wave, open-circuited studs and 31, separated by adistance equivalent to a quarter of a wave length, all measured atfrequency f The characteristic impedance Z of stubs 30 and 31 is givenby where Z is the characteristic impedance of the hybrid.

A similar analysis can be made for the short-circuited quadrature hybrid40 illustrated in FIG. 4, This analysis shows that the short-circuitedhybrid is the dual of the open-circuited hybrid and, thus, can berepresented as a shunt-connected, parallel resonant circuit. Inaddition, because the coetficient of reflection of a short circuit is l,the reflected signal at branch 2 is now equal to The negative signindicates a 180 degree phase shift. Accordingly, the complete equivalentcircuit of a short-circuited hybrid includes, in addition to a resonantcircuit, an all-pass, 180 degree phase shifter. Thus, the lumpedelementequivalent circuit of the short-circuited hybrid, as illustrated in FIG.5A, includes an all-pass, 180 degree phase shifter 50, and ashunt-connected, parallel resonant circuit 51 comprising an inductor Land a capacitor C. L and C, as before, are related by 1 ZNE and where Zis the characteristic impedance of the hybrid.

Having recognized the equivalent resonant properties of theopen-circuited quadrature coupler, and of its dual, the short-circuitedquadrature coupler, entire filter circuits can be built using onlycombinations of open-circuited quadrature couplers. The simplest filtercomprises the series combination of one open-circuited and oneshortcircuited coupler. More generally, a filter having any arbitrarynumber of sections 60, 61 62 can be made, where successive sections arealternately opencircuited and short-circuited couplers, as illustratedin FIG. 6. In general, the section impedances Z Z Z of the severalsections can assume an arbitrary values, depending upon the filtertransfer characteristics desired. In the special case where Z =Z Zm, aconstant-k type filter is obtained.

Alternatively, the dual of the filter shown in FIG. 6 can be devisedsimply by substituting for each of the couplers 60, 61 62, its dual.Thus, as illustrated in FIG. 7, a short-circuited coupler 70 issubstituted for open-circuited coupler 60, Whereas an open-circuitedcoupler 71 is substituted for short-circuited coupler 61, et cetera. Asis known, the band-pass characteristics of these two, dual filterstructures are the same.

As was noted previously, there is an all-pass 180 degree phase shiftassociated with the output signal derived from the short-circuitedcoupler. This conveniently obtained, broadband phase shift makes itrelatively simple to obtain a balanced output from an unbalanced signalsource by using two dual filters having an odd number of sections, asillustrated in FIG. 8. In this arrangement, an unbalanced signal source79 is connected to two dual filter circuits 80 and 81, each comprisingan odd number, (2n1), of sections, Where n is any integer 1, 2 n.

5 Because there is one more short-circuited coupler in filter circuit 80than there is in the dual filter circuit 81, the two output signals are180 degrees out of phase. In all other respects, however, the filtertransfer characteristics of the two filters are the same, thus providinga convenient means of obtaining a balanced output from a broadbandunbalanced signal source. This, typically, can not always be obtainedusing a conventional transformer. Thus, in all cases it is understoodthat the above-dc scribed arrangements are illustrative of a smallnumber of the many possible specific embodiments which can representapplications of the principles of the invention. Numerous and variedother arrangements can readily be devised in accordance with theseprinciples by those skilled in the art without departing from the spiritand scope of the invention.

I claim: 1. A filter structure comprising: at least one open-circuited 3db quadrature hybrid junction connected in series with at least oneshortcircuited 3 db quadrature hybrid junction. 2. The filter accordingto claim 1 wherein said hybrids are lumped-element hybrids.

3. The filter according to claim 1 wherein said hybrids are transmissionline type hybrids.

4. A circuit for deriving balanced output signals from an unbalancedsignal source, comprising: a pair of dual filters, each including:

an odd number of open-circuited and short-cio cuited 3 db quadraturehybrid junctions, connected alternately in series;

6 one end of each filter being coupled to said signal source; and thebalanced signals being obtained at the other ends of said filters.

References Cited UNITED STATES FATENTS 2,908,813 10/1959 Morrison 333l02,942,209 6/1960 Cohn 33310 3,277,403 10/1966 Cohn 333-10 X 3,337,8218/1967 Engelbrecht 333l1 X 3,435,384 3/1969 Renkowitz 333-73 OTHERREFERENCES HERMAN KARL SAALBACH, Primary Examiner W. H. PUNTER,Assistant Examiner US. 01. X.R. asp-11, 7s

Disclaimer 3,500,259.-Har0ld Seidel, \Varren Township, Somerset County,NJ. FILTER CIRCUITS USING ALTERNATE OPEN- AND SHORT-CIR- CUITED 3 dbQUADRATURE HYBRIDS. Patent dated Mar. 10, 1970. Disclaimer filed June14, 1972, by the assignee, Bell Telephone Laboratories, Incorporated.Hereby enters this disclaimer to claims 1 and 3 of said patent.

[Oflicz'al Gazette January 16, 1.973.]

